This invention relates to a phase synchronizer or synchronizing device for use in phase synchronizing a reference carrier signal indispensable in demodulating in a receiver a combined amplitude and phase modulated signal derived by quadrature modulation of those codes which are assigned to a plurality of signal points, N in number, arranged on an X-Y complex amplitude plane.
As a polyphase modulation system wherein a finite number of those signal points are defined on an X-Y complex amplitude plane which are assigned to codes to be selectively transmitted on a reference and a quadrature carrier signal, a system is already known wherein the signal points are arranged on the circumference of a circle as described by William R. Bennett and James R. Davey in detail in a book, Data Transmission, published in 1965 by McGraw-Hill Book Co., Inc. As set forth at pages 201-209 of the referenced book, use may be made, in carrying out the demodulation, of differential phase detection for which it is unnecessary to reproduce or recover the reference carrier signal. This is unnecessary because the transmitted signal includes no amplitude information.
With a recent improvement in the quality of transmission channels, combined amplitude and phase modulation systems have come into practical use wherein the positions of the signal points are no longer restricted to the circumference of a circle but are arranged optionally as disclosed by D.N. McRae in "Performance Evaluation of a New Modulation Technique," published in IEEE Transactions on Communication Technology, Vol. COM-19, No. 4 (August 1971) at pages 431-445. For this system wherein the transmitted codes have both phase and amplitude information, it is mandatory in a receiver to reproduce a reference carrier signal for use in demodulating the transmitted codes. In the receiver, a quadrature carrier signal is also produced by shifting the phase of the reproduced carrier signal by ninety degrees. These two carrier signals are multiplied by the amplitude and phase modulated signal to provide first and second products, respectively. Based on the principles of coherent detection, the first and second products provide X (real) and Y (imaginary) components of the modulation signal. A demodulator for such demodulation is known as a quadrature phase and amplitude demodulator. This system has problems in reproduction of the reference carrier signal, namely, difficulties in detecting a phase difference between the reproduced carrier signal and an incoming reference carrier signal implicitly contained in the modulated signal supplied to the receiver. In other words, this system requires in the receiver a complex phase error detector like the carrier phase detector illustrated in FIG. 4 on an article contributed by T. Furuya et al to the Third International Conference on Digital Satellite Communication, November 1975, entitled "C-3 Hybrid Modem for 120 Channels FDM Telephony Signal Transmission" (pages 91-98).